Pickling bath for stainless steel and process for the preparation thereof



I l l g l i 5 l i t; m l \r I.

United States Patent 3 Claims. (Cl. 252-101 This invention relates to an inorganic acid pickling bath for stainless steel, containing, in addition to acid, ammonium ions in determined proportions.

I have investigated the pickling of stainless steel, endeavoring to attain more satisfactory technological results than are obtained by the commonly employed baths, particularly in respect to the pickling rate, the metal etching, the surface appearance after treatment, and ease of employment of the bath itself.

Difliculty is encountered in pickling stainless steel mate'rial, owing to the resistance to reaction with acids exhibited by the surface crust formed of scales, annealing oxides, and working residues deposited in the manufacturing process.

Because of resistance to acids, pickling in two different and distinct baths is generally recommended. The first bath aims at swelling and softening the crust, the second at reacting with and removing the coat-ing of scales and working residues.

Baths employed for softening and swelling the crust are made up of sulfuric acid or of sulfuric-hydrochloric mixtures, whereas the ones designed to remove said crust are constituted of various mixtures of acids to which hydrofiuoric acid is added. In any event, either the selection of the baths, or the concentration of the components thereof, and the temperature of operation should always be determined as a function of the crust constitution, of its consistency, and also of the type of steel being treated.

Generally, a first pickling in 20% H SO is recommended for stainless steel, whereby the crust of oxides and residues is softened and swelled. Subsequently a second pickling in a mixture of acids is carried out, which acids are capable of reacting with the scale film and then removing it.

The research which gave rise to the instant invention hereinafter described, started when treating the steel material with two pickling baths having distinct functions, as stated above.

Sufliciently good results were obtained employing the following cycle of operations on AISI 316 stainless steel:

(1) Pickling in H 80 at 50-60 C.;

(2) Washing in water;

(3) Pickling in 20% HNO -4% HF at 50-60 C.; (4) Washing with water;

(5) Passivation;

(6) Drying.

Since the employment of two pickling baths, besides requiring more equipment, also causes a slowing down in the industrial productive cycle, the formulation of a single pickling bath was first considered, which bath should allow at least equivalent results to be attained.

A number of sulfo-nitric-fiuorhydric mixtures at different concentrations were tested, while taking care that the surface crust is entirely removed, and at the same time always keeping the amount of material etched away within sufiiciently narrow limits.

In the tests it was noted that the crust that was more 3,230,172 Patented Jan. 18, 1966 resistant to the action of baths was always the one on the inner surface of the steel material. If the piece was kept insistently in the bath, almost always this resulted in enhanced etching together with incomplete pickling.

Complete pickling is obtained with the following bath:

H 80 (98% )20% by weight; 246 g./lt. HNO (65% )-20% by weight; 246 g./lt. HF (40% )-6% by weight; 74 g./lt. H 054% by weight; 665.2 g./lt. Temperature-50 C.

After treatment, the material, A181 316 steel, had a clean appearance, the etching (corrosion attack) was kept within reasonable limits, and an appreciable reduction in pickling duration occurred.

In carrying out the tests, it was found, unexpectedly, that the presence of ammonium ions in a bath of this type further improves the operational capacities thereof, either in respect to pickling times, or in the final appearance of the treated material, which appears very light and semibright, or in respect to the amount of steel material etched away. The last is diminished, owing to the lesser time required for the pickling.

Ammonium ions may be introduced into the bath by adding ammonium difiouride and/or ammonia or other ammonium salts. In this way, handling the hydrofluoric acid is avoided. Some of the above-mentioned substances may be added, together with ammonium difluoride, when the amount of ammonium ions introduced with the difluoride is insufficient to obtain the desired ammonium ion concentration.

The effect of introduction of ammonium salts into the bath proved to be bound with an optimum range of concentrations, outside of which range the bath loses all its advantages, such as the speed of pickling, and the surprising excellent influence on the final appearance of the treated material.

The optimum range of the concentration of ammonium ions necessary in order to preserve to the bath its characteristics is comprised between 2 and 10% Hydrochloric acid exerts a negative influence on the final appearance of treated material. An addition to a bath based on sulfuric, nitric, hydrofluoric acids, and ammonium salts, of HCl in the amount of 5% gives rise to the formation of surface dark coating on the treated pieces.

The preferred compositions for pickling baths according to this invention are comprised within the following limits of concentration of each component:

H 50 (98%)from 15 to 35% by weight; from 169.7

to 508.9 g./lt.

HNO (65%)--from 5 to 20% by weight; from 56.5

to 290.8 g./lt.

NH HF -frorn 5 to 15% by weight; from 56.5 to 218.1

g./lt.

(NH,) SO,from 0 to 10% by weight; from 0 to 145.4

g./lt.

H O-from to 20% by weight; from 848.6 to 290.8

The fluoride content introduced by the NH HF above corresponds to a concentration of 25-10% of HF.

A 'bath of this kind is readily prepared by adding the ammonium salt alone, or a mixture of ammonium salts, to the aqueous solution of acids. The sequence of introduction of the components may be varied without impairing the efliciency of the bath.

The temperature of operation is preferably about 50 C.

3 The advantages readily attainable are summarized as follows PICKLING RATE In comparison with normal baths, a decrease in required pickling time may be obtained of from 60 to 80%, depending on the material.

CORROSION The treated steel material undergoes a very uniformly spread etching, without preferential attack zones such as is often observed with many baths. improvement is due to the good pickling rate which minimizes the aggressive action of acids in zones of black oxide crust which has less consistency and is more easily attackable.

As a result of the faster pickling rate, the baths of the present invention induce on the treated material an etching which is less than that observed on the same material treated with conventional baths.

MATERIAL APPEARANCE The final appearance of the treated material is appreciably better than that obtainable by conventional pickling baths, because surfaces of very light and semibright aspect are obtained.

BATH EFFICIENCY No slackening of pickling rate is observed until there is an iron content of more than 20 g./l. in this connection, it is noted that the currently employed baths (HNO -HF) are discharged at 16-18 g./l. of Fe. In the instant invention, good rates can be maintained by suitable additions of any lacking components.

PREPARATION In preparing the bath, the hand-ling of HF is avoided, since it involves risks and requires skilled labor.

For a better understanding of the invention, a number of examples are given.

In Example 1, the composition of a pickling bath of the type according to this invention is indicated, together with results obtained in comparison with a conventional pickling bath.

In Example 2, the aging behavior of a bath prepared according to the instant invention is reported.

[It is to be understood that the components, proportions and conditions employed in the following specific examples are in no way limitative of the scope of this inven-tion. Example 1 A pickling bath having the following composition is prepared by pouring the acids into water and subsequently adding the mixture of ammonium salts:

COMPOSITION B H 50 (98% )25% by weight; 323 g./1t.

HNO (65% 13% by weight; 167.9 g./lt.

NH HF 10% by weight; 129.2 g./1t.

(NH SO 5% by weight; 64.6 g./1t.

H O47% by weight; 607.2 g./-lt.

For A, the pickling times are the sum of the dipping times in the two baths.

The pickling duration varies, depending on the material being treated, and on the constitution and amount of 0.25 sq. m. of material surface were treated, per liter of bath, before evidencing a decrease in pickling rate, while reaching an iron content of 20 g./l., and a consumption of 3% of 'NH HF in comparison with the starting amount.

It is to be noted that the aging test was carried out on hot rolled pipes. With cold drawn and reheated pipes larger values of treated surface per bath liter are attained. In consequence, a higher iron content is reached before a decrease in pickling rate can be noticed.

When conventional pickling baths are used, a decrease in pickling rate is observed when the iron content reaches the limit of 15-16 g./l.

I claim:

1. A pickling bath for stainless steel consisting of a water solution consisting essentially of 15-35% sulfuric acid, 5-20% nitric acid, 2.5-10% hydrofluoric acid, and ammonium ions in amounts between 2 and 10% of the total weights of the acids, and water, said ammonium ions being supplied from at least one member of the class consisting of ammonium sulfate, ammonia, and ammonium difiuoride.

2. A pickling bath for stainless steel consisting of a water solution consisting essentially of 15-35% sulfuric acid, 15-20% nitric acid, 2.510% hydrofluoric acid, and 5-15 ammonium difluoride, and water.

3. A process for pickling a stainless steel article comprising contacting said article in a single pickling dipping operation with a pickling bath consisting of an aqueous solution consisting essentially of 15-35% sulfuric acid, 520% nitric acid, 2.5-10% hydrofluoric acid, and containing between 2 and 10% of ammonium ions based on the total weight of said acids, said ammonium ions being formed from at least one member of the class consisting of ammonium sulfate, ammonia, and ammonium difluoride.

References Cited by the Examiner OTHER REFERENCES Handbook of Chemistry, Lange (1939), pages 584- 589.

JTULIUS GREENWALD, Primary Examiner,

ALBERT T- M xa iner, 

1. A PICKLING BATH FOR STAINLESS STEEL CONSISTING OF A WATER SOLUTION CONSISTING ESSENTIALLY OF 15-35% SULFURIC ACID, 5-20% NITRIC ACID, 2.5-10% HYDROFLUORIC ACID, AND AMMONIUM IONS IN AMOUNTS BETWEEN 2 AND 10% OF THE TOTAL WEIGHTS OF THE ACIDS, AND WATER, SAID AMMONIUM IONS BEING SUPPLIED FROM AT LEAST ONE MEMBER OF THE CLASS CONSISTING OF AMMONIUM SULFATE, AMMONIA, AND AMMONIUM DIFLUORIDE. 